This invention relates to an index type color picture tube, and more particularly a composition of phosphors constituting the fluorescent screen of the picture tube.
As is well known, the fluorescent screen of index type color picture tube generally comprises tricolor stripe shaped phosphor layers and a stripe shaped index phosphor layer adapted to show the position of a scanning beam. Used as the tricolor phosphors are a Y.sub.2 O.sub.3 :Eu phosphor for red color, a ZnS:Cu, Al or ZnS:Cu, Au, Al phosphor for green color, and a ZnS:Ag phosphor for blue color. Used as the index phosphor are a YAlO.sub.3 :Ce phosphor (for emitting ultraviolet rays) having a short after-glow time, a Y.sub.3 Al.sub.5 O.sub.12 :Ce phosphor (emitting green color) or a Y.sub.2 SiO.sub.5 :Cu phosphor (emitting ultraviolet rays), etc. However, with zinc sulfide type phosphors emitting green and blue colors, their brightness will generally increase substantially linearly with increase in the electron beam current within its low value range, but within the high value range, their brightness will saturate without exhibiting linear increase. Only with the conventional red color phosphor, even in the high current range, there occurs no appreciable saturation phenomenon of the brightness, and the brightness increases linearly as the electron beam current increases.
For this reason, in a color picture tube utilizing such tricolor phosphors, a white color picture becomes more reddish in the high current range than in the low current range, thus degrading uniformity of whiteness. With a shadow mask type color picture tube having three electron guns corresponding to the tricolor phosphors, this problem can be solved more simply by correcting characteristics of the associated electric circuits, but in the index type color picture tube having no shadow mask but having a single electron gun, it is quite difficult to take advantage of such a remedy. Thus, practical studies of green color phosphor and blue color phosphor having less saturation of brightness even in the high current range have been so far made, but those equivalent to the said phosphor of zinc sulfide system in the brightness and emission tone have not been obtained up to now.
To improve the brightness in the red color phosphor, it has been proposed to add about 5 to 30 ppm of Tb thereto [J. Electrochem. Soc. Vol. 123, No. 1, pages 75-78 (January, 1976); J. Electrochem. Soc., Vol. 126, No. 2, pages 305-312 (February, 1979)].
The applicants likewise proposed an index type color picture tube using a Y.sub.2 O.sub.2 S:Eu phosphor containing 0.01 to 3 ppm of Tb as a red color phosphor (Japanese patent application No. 58-123207; corresponding U.S. patent application Ser. No. 62,776, filed July 5, 1984now U.S. Pat. No. 4,625,147), where, contrary to the conventional concept that the characteristics of green color phosphor and blue color phosphor are made to approach those of the red color phosphor, the applicants tried to obtain a red color phosphor having a brightness saturation characteristic on the same level as those of the green color phosphor and the blue color phosphor, and succeeded in obtaining a remarkable brightness saturation characteristic by controlling the Tb content to a few ppm (Tb has been so far added to the red color phosphor in an amount of about 5 to 30 ppm to improve the brightness of the red color phosphor as described above). However, the applicants have found that it is practically difficult to obtain a red color phosphor whose impurity Tb content is exactly controlled to such a very small range as to directly show the brightness saturation characteristic on the substantially same level of those of the green color phosphor and the blue color phosphor to be used together in combination, owing to fluctuations in the contents of impurities in the phosphor raw materials, etc.